Aqueous emulsion polymers or latexes in both clear and pigmented form are well-known. Examples of their uses include interior and exterior architectural coatings, general metal coatings, adhesives, and the like. Latexes can be formed by aqueous emulsion polymerization of ethylenically unsaturated monomers such as styrene, butyl acrylate, methyl methacrylate, vinyl acetate, vinyl 2-ethylhexanoate, acrylic acid, acrylonitrile, glycidyl methacrylate, 2-hydroxyethyl acrylate and the like. When ethylenically unsaturated monomers that contain a polar functionality such as the carboxyl group in acrylic acid, and/or the hydroxyl group in 2-hydroxyethyl acrylate, are used, there is a propensity for these groups to be found at or near the surface of the emulsion particles because of the affinity of these groups for the aqueous environment. In addition, techniques for increasing the amount of any non-water reactive functional group near the surface of the emulsion particles are known to those skilled in the art of emulsion polymerization. Illustrative of such techniques is the production of a core and shell latex in which the core of the particles has a given composition that may contain a small amount of the functional groups or be devoid of them and the shell or outer layers of the particles have a different composition which may be rich in the functional groups. The large number of independent variables which can be manipulated in designing latex particles renders the preparation of improved latexes problematic.
This invention as shown below, illustrates the importance of polymer gel fraction, polymer swell ratio, and the importance of choosing the appropriate monomers for allyl survival in water-borne particles. In addition, polymer film properties such as gel fraction, swell ratio, molecular-weight between crosslink points, and solvent resistance (methyl ethyl ketone double rubs) are reported. Film properties can be greatly improved if the polymer film can be crosslinked after film formation.
The preparation of emulsion polymers containing "dangling" or pendant double bonds is one possible solution for a coating system which crosslinks ambiently, thermally, and photochemically. However, this is difficult to achieve since the emulsion polymerization process would be expected to also involve free-radical polymerization with such double bonds. While emulsion polymers allow one to prepare high molecular weight polymers at low viscosity, water-borne polymer particles which contain unsaturation allow one the flexibility to later crosslink the coating under a variety of cure conditions. The use of "allylic" groups as vehicles to cure oil-based paints has been used in alkyd resin systems for years. Alkyds are esterified resins prepared from polyols, dicarboxylic acids, and fatty acids. Dangling double bonds in alkyd resins are contained in the fatty acid portion of the resins. Double bonds are referred to herein as "dangling" because they are contained in the side chains of the polymer, and are not contained in its backbone. The reaction of oxygen and allylic hydrogen atoms to form peroxides in the fatty acid portion of the resin starts the crosslinking chemistry. A so-called "drier" in the coating, typically, a cobalt salt, decomposes the peroxide to active hydroxyl and organic radicals at room temperature. The chain of chemical events which follows leads to a crosslinked coating. The advantage of this system is that it uses oxygen as an active agent in the crosslinking chemistry via a free-radical mechanism; thus, the coating formulation is stable until it is exposed to oxygen. Its obvious disadvantage is that organic solvents are liberated as the coating dries. Recent examples of water dispersible alkyds are described in U.S. Pat. No. 5,378,757, and references cited therein and PCT application WO 95/02019, as well as U.S. Pat. No. 4,251,406. In addition, as commercial alkyd coatings age, they have a tendency to become brittle and yellow.
In addition to alkyds, unsaturated polyesters are another class of resins which are prepared from the esterification of unsaturated dicarboxylic acids, and polyols. The unsaturated moiety of the polymer is usually located in the backbone of the polymer molecule. Although unsaturated polyesters can be oil-in-water emulsions, water-in-oil emulsions of unsaturated polyesters are typical. U.S. Pat. No. 3,629,169 describes a curable water-in-oil emulsion in which the oil phase contained a carboxylic acid-containing unsaturated polyester, a copolymerizable monomer such as styrene or methacrylic ester, non-ionic emulsifiers, a thixotropic thickener, a basic neutralizing agent, a promotor-catalyst system composed of dimethyl aniline, an oil soluble cobalt salt, and a thermal peroxide initiator. An emulsion was obtained by adding water while the oil phase was being stirred.
U.S. Pat. No. 3,950,287 describes a process for converting polymerizable water-in-oil emulsions to thermoset objects at room temperature. The oil phase consists of a carboxylic acid-containing unsaturated polyester resin, polymerizable monomers, oil-soluble cobalt salts, and a promoter consisting of N-hydroxyl alkyl substituted, phenyl-substituted tertiary amines and dialkyl anilines. The water phase, which contains hydrogen peroxide, is stirred into the oil to obtain an emulsion which rapidly cures. This reference claims that the effectiveness of a combination of catalyst and promoter is enhanced when it includes a mixture of tertiary amines where one of the tertiary amines has a hydroxyalkyl group attached to the amino group of the nitrogen, and the second amine is a dialkyl aniline. It is claimed that the --OH group of hydroxyalkyl substituted amines coordinates with cobalt which enhances the effectiveness of the catalyst-promotor system in affecting rapid gelation of the emulsion. The emulsions in the above inventions are claimed to be useful for preparing shaped objects.
U.S. Pat. No. 4,077,931 describes a process for producing a pourable (or pumpable) thermosetting water-in-oil emulsion comprising water as the continuous phase, and an unsaturated polyester, a copolymerizable monomer, and a peroxide-cobalt-tertiary amine curing system in the oil phase. This reference teaches that unsaturated polyesters with molecular weights greater than 1800 g/mole give a water-in-oil emulsion with superior properties. This reference further described a formulation recipe which allowed the emulsion to be shaped into usable objects before substantial cure of the emulsion occurred. Thus, this reference explored the use of water-in-oil emulsion prepared from unsaturated esters as molding compounds for shaped objects.
An approach to an emulsion particle which has dangling double bonds, which avoids the problem of polymerization of such double bonds during emulsion polymer synthesis is to first carry out a synthesis providing a latex particle with acid, epoxy, or carbodiimide functionality which is followed by reaction with an epoxy or acid functional compound, respectively, which contains a double bond.
U.S. Pat. No. 4,244,850 describes air drying and air curable latex coating compositions that have unsaturated resin particles dispersed in the water phase and which are combined with an emulsion of a drier salt. The coating compositions were formed by first preparing a latex whose particles contain either carboxyl or 1,2-epoxide functionality. After formation of the latex, a portion of the reactive carboxyl or 1,2-epoxide functionality was reacted with an ethylenically unsaturated monomer carrying one of the other of said reactive groups to provide unsaturated groups on the latex particles. For example, if the initial latex is made with free carboxylic acid functionality, it is then later reacted with glycidyl acrylate, and if the initial latex is made with free 1,2-epoxide functionality, it is then later reacted with methacrylic acid to leave free ethylenic unsaturation on the surface of the modified latex particle. The emulsion of a drier salt is then added after the post reaction. Drier salts such as cobalt naphthanate are used to facilitate the air curing of the residual acrylate moiety. Interestingly, no analytical data is given to support the claims. Instead, only broad observations of cast films are given. For example, the patent states that "water resistance, hardness and adhesion were observed, and it was seen that these gradually improved to possess the properties of cured alkyd coatings." No controls were performed.
U.S. Pat. No. 4,033,920 describes acrylic dispersion polymers which contained dangling methacrylate double bonds. In the process described in this reference, glycidyl methacrylate was reacted with carboxylic acid-containing polymers in solution. The solvent was removed under reduced pressure and replaced by a polymerizable monomer. The polymer solutions were neutralized with ammonia, diethanolamine, or triethyl amine, and then dispersed into water to form an emulsion. This reference suggests that the oil phase of the emulsion could contain conventional initiators such as benzoyl peroxide, and azobisisobutyronitrile. For photocure, photosensitizers such as benzoin (only one example was demonstrated), benzoin methyl ether, and the like were suggested. This reference was directed to the use of these emulsions for preparing films which cured by ultraviolet light or electron beam.
A second approach is to react carboxylic acid-containing latex particles with carbodiimide methacrylates to produce latex particles with dangling methacrylates (see U.S. Pat. No. 5,371,148.)
U.S. Pat. No. 3,219,610 teaches that emulsion particles prepared from allyl methacrylate, ethyl acrylate, and methyl methacrylate produced acetone insoluble films when they were baked at 150.degree. C. for 30 minutes, whereas, films air dried for two hours were soluble in acetone. No analysis of the allyl content of these films were made to determine the amounts if any, of allyl moieties which survived the emulsion polymerization process. No styrene was used in this process, and no morphology design of the latex particles was attempted. No methyl ethyl ketone (solvent) resistance data was given.
L. V. Molkova, et al., Polymer Science USSR 28 (2) p. 321 (1986) describes emulsion polymerization of copolymers of allyl methacrylate and methyl methacrylate. This reference teaches allyl survival, as measured by bromination, as high as 83%. The glass transition temperatures of the copolymers were greater than 80.degree. C. Crosslinking occurred when the film was heated at greater than 150.degree. C. in the presence of a radical initiator.
GB Patent No. 2206591A taught the use of allyl methacrylate to prepare seed/shell/mantle latexes where the shell (as used below, the seed/shell would be referred to as the core) was crosslinked with allyl methacrylate. More recently, Lee and Rudin (Makromol. Chem., Rapid Commun. 10, 655 (1989)) used 5 weight percent of allyl methacrylate in the core of a core/shell latex particle as a "crosslinking agent" to give the core gel content. U.S. Pat. No. 4,925,893 and European Patent Application 0 297 781 A2 teach the preparation of latexes using a combination of allyl methacrylate and diallyl phthalate with other polymerizable monomers to prepare coatings which cure ambiently in the presence of catalyst. Styrene is disclosed as a possible but not preferred monomer.
U.S. Pat. No. 5,264,482 teaches the reaction of allyl glycidyl ether with carboxylic acid-containing copolymers to produce a water-soluble polymer with a "dangling allyl ether group" which was used as a curative additive to latex paints to increase lap time (or improve wet edge).
Although there is a need for products such as those described above, there is a further need for advanced products that have improved, lower initial molecular weight characteristics, improved adhesion, and products that will crosslink under ambient conditions or low to moderate temperatures in the presence of air with or without the use of drier salts or fugitive filming aids.